1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor. Further, the present invention relates to a method for manufacturing a semiconductor device having the thin film transistor. As an example of the semiconductor device, a display device is given, in particular. As an example of the display device, a liquid crystal display device and an EL display device are given.
2. Description of the Related Art
In recent years, thin film transistors formed using a semiconductor thin film (having a thickness of about several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface (e.g., a glass substrate) have been attracting attentions. Thin film transistors are widely used for ICs (integrated circuits) and electronic devices such as electrooptical devices. In particular, thin film transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices and the like. In an image display device such as a liquid crystal display device, a thin film transistor using an amorphous semiconductor film or a thin film transistor using a polycrystalline semiconductor film is mainly used as a switching element.
The thin film transistor using an amorphous semiconductor film has low mobility, i.e., low current-driving capability. Thus, when a protection circuit is formed using a thin film transistor formed using an amorphous semiconductor film, a large-sized thin film transistor should be formed as a countermeasure against electrostatic breakdown, which leads to hindrance to narrower frame parts. In addition, if a large-sized transistor is formed, parasitic capacitance between a scan line electrically connected to a gate electrode and a signal line electrically connected to a source electrode or a drain electrode may increase, and thus power consumption may increase, which is problematic.
On the other hand, a thin film transistor using a polycrystalline semiconductor film has advantages in that its mobility is greater than that of a thin film transistor using an amorphous semiconductor film by two or more digits, and a pixel portion of a liquid crystal display device and peripheral driver circuits thereof can be formed over the same substrate. A process of the thin film transistor using a polycrystalline semiconductor film, however, is more complicated than that of the thin film transistor using an amorphous semiconductor film, because of crystallization of a semiconductor film and addition of an impurity element (doping). Therefore, there is a problem of low yield and high cost. As a method for forming a polycrystalline semiconductor film, there is known a technique in which a pulsed excimer laser beam is shaped into a linear beam by an optical system and an amorphous semiconductor film is scanned to be irradiated with the linear beam so that the amorphous semiconductor film can be crystallized.
In addition, as a switching element of an image display device, a thin film transistor using a microcrystalline semiconductor film is known, as well as a thin film transistor using an amorphous semiconductor film or a thin film transistor using a polycrystalline semiconductor film (e.g., see Patent Document 1). As a method for manufacturing a thin film transistor using a microcrystalline semiconductor film, a technique is known in which an amorphous silicon film is formed over a gate insulating film, a metal film is formed over the amorphous silicon film, and the metal film is irradiated with a diode laser in order to modify the amorphous silicon film into a microcrystalline silicon film. With this manufacturing method, the metal film formed over the amorphous silicon film only converts light energy of the diode laser into thermal energy and is removed in a later step. That is, in this method, the amorphous silicon film is heated only by conduction heating from the metal film and the microcrystalline silicon film is formed by this heat (e.g., see Non-Patent Document 1).
In a process for manufacturing the aforementioned thin film transistor, reduction in the number of photomasks used in photolithography is important for simplification of the process. For example, if one photomask is added, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like and, moreover, steps before and after the aforementioned steps, such as film formation, etching, resist removal, cleaning, drying, and the like. The number of steps is significantly increased only by adding one photomask in the manufacturing process. Therefore, many techniques for reducing the number of photomasks in a manufacturing process have been developed. By employing a channel-etch type thin film transistor, the number of photomasks can be reduced.
However, in a channel-etch type thin film transistor, a back channel portion is exposed, which causes leakage current. Thus, it has been difficult to make off current sufficiently low. Therefore, a variety of devices has been made; for example, plasma treatment is performed on the back channel portion (e.g., see Patent Document 2).